An ATP-independent activity which catalyzes the transfer of one strand from a linear duplex DNA molecule to a complementary circular single strand has been detected in crude extracts from both mitotic and meiotic yeast cells. The assay requires the addition of yeast single-stranded DNA binding protein (ySSB). The polypeptide (yeast Strand Transfer Protein alpha ySTP alpha) responsible for this activity has been purified to homogeneity from meiotic cells, characterized, and antibodies raised from a rabbit. Using the antibodies, the gene for ySTP alpha has been isolated, its nucleotide sequence determined, and its regulation studied. Although the gene is not essential for mitotic cell growth, it is required for meiotic homologous recombination and sporulation, proving that ySTP alpha is one of the meiotic recombination components in yeast and that the ATP-independent reactions catalyzed by ySTP alpha are biologically important. The ySTP alpha mRNA and poly-peptide are constitutively expressed in both mitotic and meiotic Yeast cells. However, the polypeptide is uniquely activated during meiosis by a mechanism that has not yet been identified. In addition, one of the yeast DNA repair genes, RAD18, has been cloned, its nucleotide sequence has been determined, its polypeptide overproduced and purified, and its regulation studied.